The Rhythm Of The Heart: Measuring Heart Rate Recovery In Twins

Heritage or environment: Why does the heart recover faster after exercise in some people but not in others? Scientists found an answer when pushing twins to their limits.

The heart beats 24/7, lifelong. It constantly pumps blood into the body and supplies organs and tissue with oxygen and nutrients. Its working speed depends, among other things, on what you are doing at the same time; it beats slower during sleep, but once you start walking, it increases its pulse and sometimes even starts cantering, because now the muscles require much oxygen-saturated blood. To return to rest after such efforts requires some time. This period is called heart rate recovery (HRR), or heart recovery frequency.

The vagus nerve is mainly responsible for the recurrence of peace of the heart muscle. It is also called the recovery nerve because it is capable of reducing the heart rate and keep it low in times of rest. Normally, the heartbeat eases faster in people that move a lot; the heart frequency of an Olympic runner will return to resting frequency much faster than that of an untrained human that avoids sweating. But also among athletes, well-trained and poorly-trained ones, occasional sportspersons and exercise sourpusses, there are differences.

These exact deviations were the focus of interest of a group of researchers from The Netherlands when they called hundreds of twin pairs for a fitness test. In their study, Ineke Nederend and her colleagues from the Vrije Universiteit Amsterdam could show (1) that individual differences of HRR and the recovery response of the vagus nerve were predominantly determined by heritability.

The heart rate is important in science because it affects health. Studies have not only shown that a low heart rate frequency at rest is associated with lower mortality, but also that fast recovery after stress-induced heart rate protects people — statistically — from premature death, especially in those older than 45 years.

Two groups of factors account for the individual differences in HRR: genes and the environment. Environmental factors include all those that are shared by family members, e.g. the place of residence. However, there are also individual environmental factors; when one child uses its bicycle to ride to school while all others do not, this marks a difference besides common growing-up conditions. Twin studies now allow scientists to unravel which of the factors exhibit the largest influence and which are less relevant.

In the paper, scientists argue that when the similarities in HRR are larger in monozygotic (MZ=identical) twins than in dizygotic (DZ=non-identical) twins, this supports the view of a larger influence of the genes, since the genetic outfit on MZ twins is almost 100% identical, while DZ twins share only about 50% of their genes. If, however, MZ and DZ twins do not differ much in a specific characteristic, then common, e.g. familial, factors exhibit the largest influence on this characteristic.

A total of 491 twins aged 16 to 18, and 37 siblings of these twins, participated in the study. All had to answer questions related to their bodily activity behaviors. What kind of sports they engage in? How long and how often? From these, the scientists calculated a grading, the so-called MET (metabolic equivalent task) score.

Subsequently, all twins had to perform on an ergometer bike to their individual limits (exhaustion). Scientist noted the HRR after sixty seconds and three minutes. To explore the influence of the vagus, they also measured the so-called respiratory sinus arrhythmia (RSA), the frequency modulation of the heart-beat induced by respiration. During inspiration, the frequency increases, while during expiration it decreases; this modulatory activity is mediated by the vagal nerve.

When the researchers evaluated these data, they indeed could identify a large influence of the genes. For the differences in exercise behavior, they calculated a genetic contribution of 80%, thus, it is dependent on genes to a rather large degree. For the heart rate at rest, this contribution of genes was found to be 66%, 60% for HRR after 60 seconds, and 65% for HRR after 3 minutes, so still a dominance of genes over environmental influence. Heritability of RSA at rest was found to be 58%, but that after 60 seconds only about 23%. The remaining differences to 100% are attributable to environmental factors.

And something else was seen by the scientists: Most likely different sections of the human genome (the set of all genes) are responsible for the control of the vagal nerve over heart rate frequency on the one hand, and the first recovery after an exercise on the other.